1. Field of the Invention
The present invention relates to Voice Over IP telephony; more particularly, the present invention relates to transport of common channel SS7 signaling messages (e.g., Message Transfer Part 3 (MTP3)) by a signaling gateway, received from a circuit switched network node having a prescribed point code, to a call agent according to a prescribed Voice Over IP telephony protocol such as MTP3 User Adaptation Layer (M3UA) or SCCP User Adaptation (UA) Layer (SUA).
2. Description of the Related Art
Common Channel Interoffice Signaling (CCIS) networks provide out of band signaling for telecommunications networks such as public switched telephone networks. Most of the signaling communications for telephone networks utilize Signaling System 7 (SS7) protocol. An exemplary SS7 compliant CCIS network includes Service Switching Points (SSPs) (i.e., an SS7 capable telephony switch), Signaling Transfer Points (STPs), and data links between the STPs and SSPs and various telephone switching offices of the network.
As recognized in the art, the hardware and software operations of the SS7 protocol are divided into “layers”, similar to the Open Systems Interconnect (OSI) Network Model specified by the International Standards Organization (ISO). The “lowest levels” of the SS7 protocol include the Message Transfer Part (MTP) Level 1, Level 2, and Level 3. MTP Level 1 and Level 2 are equivalent to the OSI Physical Layer and the OSI Data Link layer, respectively. MTP Level 3, equivalent to the OSI Network Layer, provides message routing between signaling points in the SS7 network, and re-routes traffic away from failed links and signaling points and controls traffic when congestion occurs.
SS7 messages (also referred to as signal units) are routed throughout the SS7 network based on point codes specified within the SS7 message. In particular, each node of the signaling network is assigned a prescribed point code for purposes of addressing signaling messages throughout the SS7 network. The point code includes components that represent a network hierarchy based on the protocol being deployed.
One type of signal unit, known as a Message Signal Unit (MSU), includes a routing label which allows an originating signaling point to send information to a destination signaling point across the network. The routing label includes an originating point code (OPC) specifying the originating signaling node, a destination point code (DPC) specifying the destination for the SS7 messaging packet, and a signaling link selection (SLS) field. Hence, the selection of outgoing link is based on information in the DPC and SLS.
The size of the point code may vary depending on protocol; for example, each North American point code according to the American National Standards Institute (ANSI) uses 24 bits, whereas each point code specified by the International Telecommunication Union (ITU) uses 14 bits. In particular, an ANSI point code specifies a network hierarchy based on network, cluster, and member octets (e.g., 245-16-0 decimal). ITU-T point codes are pure binary numbers which may be stated in terms of zone, area/network, and signaling point identification numbers. For example, the point code 5557 (decimal) may be stated as 2-182-5 (binary 010 10110110 101).
The Internet Engineering Task Force (IETF) Signaling Transport (Sigtran) Working Group has been addressing the transport of packet-based PSTN signaling over IP Networks, including evaluation of functional and performance requirements of the PSTN signaling. For example, IP networks need to transport signaling messages such as Q.931 or SS7 ISUP messages, received from the PSTN, between IP nodes such as a Signaling Gateway and Media Gateway Controller or IP-resident databases.
Three published proposals for transport of signaling messages include the Request for Comments (RFC) 2719 by Ong et al., “Framework Architecture for Signaling Transport”, December 1999, available on the World Wide Web at the address http://www.ietf.org/rfc/rfc2719.txt and RFC 3332 by Sidebottom et al., “Signaling System 7 (SS7) Message Transfer Part 3 (MTP3)—User Adaptation Layer (M3UA)”, September 2002, available on the World Wide Web at the address http://www.ietf.org/rfc/rfc3332.txt, and the IETF Draft by Loughney et al., “Signalling Connection Control Part User Adaptation Layer (SUA)”, Jun. 30, 2002, available on the World Wide Web at the address http://www.ietf.org/internet-drafts/draft-ietf-sigtran-sua-14.txt, all three disclosures of which are incorporated in their entirety herein by reference.
RFC 2719 specifies a Voice over IP architecture, including interactions between signaling gateways (SGs), media gateways (MGs) and media gateway controllers (MGCs). In particular, media gateways terminate media streams carried by bearer channels of Switched Circuit Networks (SCN). Media gateways also packetize media data, if necessary, and deliver packetized data to the IP network. MGCs, also referred to as call agents, manage the resources of the MGs by executing application server processes (ASPs): according to RFC 2719, the MGC serves as a possible termination and origination point for SCN application protocols, such as SS7 ISDN User Part (ISUP) and Q.931/DSS1. An SG, also referred to as an IP Transfer Point (ITP), is a signaling agent that receives/sends SCN native signaling at the edge of the IP network. The SG function may relay, translate or terminate SS7 signaling in an SS7-Internet Gateway. The SG function also may be co-resident with the MG function to process SCN signaling associated with line or trunk terminations controlled by the MG (e.g., signaling backhaul).
RFC 3332 specifies the Stream Control TransmissionProtocol (SCTP) as atransportprotocol for transporting SCN-based signaling messages (e.g., SS7) over IP. In particular, the SCTP provides a mechanism for transporting SS7 MTP3-User Part messages (e.g., ISUP, Signalling Connection Control Part (SCCP), Telephone User Part (TUP), etc.) between an SG and an MGC. Hence, an SG implementing SCTP will terminate SS7 MTP2 and MTP3 protocol layers, and deliver ISUP, SCCP and/or any other MTP3-User protocol messages, as well as certain MTP network management events, over SCTP transport associations to MTP3-User peers in MGCs or IP-resident databases.
The Internet Draft by Loughney et al. specifies SUA as a protocol for the transport of any Signalling Connection Control Part-User signalling (e.g., Transaction Capabilities Protocol, Radio Access Network Application Protocol, etc.) over IP using the Stream Control Transmission Protocol.
FIG. 1 is a block diagram illustrating an exemplary Voice over IP telephony network 10, deployed according to RFC 2719, RFC 3332, and SUA protocol, for communications with a TDM-based circuit switched public telephony system 12. The telephony system 12 includes TDM switches 13a and 13b having point codes 1.2.1 and 1.2.2 respectively. The TDM switches 13a and 13b may be substituted with mated pairs of signaling transfer points (STPs). The IP-based telephony network 10 includes signaling gateways (SGs) 14a and 14b having point codes 2.1.1 and 2.1.2, respectively. The Voice over IP network 10 also includes MGCs 16a, 16b, and 16c having point codes 1.1.1, 1.1.2, and 1.1.3, respectively, and an IP network 20 for transfer of signaling information between the MGCs and the SGs. Each signaling gateway (e.g., 14a, 14b) is configured for routing a signaling application protocol message (e.g., ISUP, SCCP) carried by a signaling message (e.g., 18a, 18b), from the circuit switched SS7 network 22 to a destination MGC (e.g., 16a, 16c) based on the corresponding destination point code (e.g., DPC=1.1.1, DPC=1.1.3).
FIG. 2 is a diagram illustrating in further detail the user adaptation architecture according to the Sigtran working group, including RFC 2719 and RFC 3332. Each SS7 signaling message 18 output by a signaling endpoint (SEP) 13 (e.g., a TDM switch or STP) includes a signaling application protocol message 24 (e.g., ISUP/SCCP, TCAP/MAP, TUP, etc.) and an MTP layer 26. The SG 14 includes an interworking resource 28 configured for removing the MTP layer, generating an M3UA header 30, and outputting via the IP network 20 an M3UA protocol message 32 having the M3UA header 30 and the user part message 24. The M3UA message 32 is output to the MGC (e.g., 16a) provisioned with the point code corresponding to the destination point code of the SS7 signaling message.
Concerns arise in adding additional Voice over IP-based nodes to accommodate increased traffic and call processing demands within the Voice over IP telephony network. In particular, as the capacity of media gateway controllers (i.e., call agents) supplied by vendors increases (e.g., call processing capacity, number of gateways supported, etc.) existing network engineers may wish to add additional call agent host controllers to enable the deployed network to meet increased traffic and call processing demands. However, building call agent clusters currently requires a dedicated originating point code (OPC) for each call agent pair, making the node appear as multiple switching entities.
Further, current congestion mechanisms in SS7 networks perform congestion notification on a point code (PC) basis through the use of the Transfer Controlled (TFC) message. As described above, SS7 over IP adaption layers such as M3UA and SUA enable many IP nodes (ASPs) to connect to an SS7 signalling gateway (SG), use the MTP and SCCP services and layers of the SG, and share a common point code. In these configurations, the SG must consider the congestion levels of all ASPs sharing a specific PC before a TFC can be sent for PC congestion notification. Existing approaches use a collective congestion level for the shared PC. Hence, when a MSU is received at an SG, the SG checks for congestion of the destination PC in the MSU. If the destination PC is congested, a TFC is returned to the node that sent the original MSU. The node that receives the TFC will stop sending MSUs for a period of time or until it receives an indication that the PC is no longer congested.
Sending a TFC based on collective PC congestion level could cause unnecessary traffic cessation for uncongested ASPs or ASPs that are congested at lower levels.